1. Field of the Invention
Apparatuses consistent with the present invention generally relate to a duplexer, and more particularly, to a duplexer capable of preventing interference between a transmit (Tx) filter and a receive (Rx) filter by virtue of structures of the Tx filter and the Rx filter.
2. Description of the Related Art
Recently, as mobile communication devices, such as mobile phones, are widely used, there are ongoing efforts to improve performance of the mobile communication devices and to manufacture them in small size with light weight. Accordingly, research is in progress to reduce the size and weight of mobile communication devices, while simultaneously enhancing the performance of components in the mobile communication device.
To this end, research is being conducted on performance enhancement of a duplexer which is one of the components of a mobile communication device.
A duplexer is a kind of representative element that incorporates filters. The duplexer functions to permit the efficient sharing of the same antenna in a communication system which is implemented using frequency division duplex (FDD) by properly separating signals received and transmitted via the antenna.
The duplexer largely includes a Tx filter and a Rx filter. The Rx filter receives a signal from an antenna and filters only a signal of a certain frequency band. The Tx filter filters only a signal of a certain frequency band of a signal produced in a communication device and provides the filtered signal to the antenna. Accordingly, the duplexer permits the transmission and the reception via one antenna by adjusting the frequencies passed by the Tx filter and the Rx filter.
The Tx filter and the Rx filter of the duplexer can be implemented using a film bulk acoustic resonator (FBAR).
The FBAR is manufactured such that a lower electrode, a piezoelectric layer, and an upper electrode are layered in order. When an external electric field is applied, the FBAR generates resonance. In more detail, when the electrical energy is applied to the upper and lower electrodes of the FBAR and an electric field temporally changing is induced in the piezoelectric layer, resonance occurs since the piezoelectric layer causes the piezoelectric effect which changes the electric energy into mechanical energy of an acoustic waveform. In this case, since the FBAR passes only a signal within a specific band centering on the generated resonant frequency, it serves as a band pass filter.
The FBAR can be implemented in a small size and with light weight, and is suitable for high-power applications. In addition, the FBAR is advantageous in view of compactness, manufacturing cost, and mass production.
When the filter is constructed using FBARs as mentioned above, it is possible to raise the quality factor (Q), which is one of principal properties of the filter, and to cover a wide driving frequency, e.g., from frequency bands of micro unit to personal communication system (PCS) and digital cordless system (DCS) frequency bands. Based on these features, the FBAR is attracting much attention as a component for use in a duplexer.
FIG. 11 is a block diagram of a related art duplexer using a plurality of FBARs, as disclosed in U.S. Pat. No. 6,262,637. FIG. 12 is a diagram illustrating a function of the phase shifter 70 of FIG. 11, and FIG. 13 is another diagram illustrating the function of the phase shifter 70 of FIG. 11.
Referring first to FIG. 11, the related art duplexer 10 includes an antenna port 20, a Tx port 30, and a Rx port 40. The duplexer 10 further includes a Tx filter 50 between the antenna port 20 and the Tx port 30, and a Rx filter 60 between the antenna port 20 and the Rx port 40.
The Tx filter 50 includes a plurality of first resonators FBAR11, FBAR12, and FBAR13 that are connected in series between the antenna port 20 and the Tx port 30, and a plurality of second resonators FBAR21 through FBAR22 that are formed on divided lines between the antenna port 20 and the Tx port 30 to which the Tx filter 50 is connected while coupled to the first resonators FBAR11 though FBAR13 in parallel.
Similarly, the Rx filter 60 includes a plurality of first resonators FBAR14, FBAR15, and FBAR16 that are connected in series between the phase shifter 70 and the Rx port 40, and a plurality of second resonators FBAR23 through FBAR26 that are formed on divided lines between the phase shifter 70 and the Rx port 40 to which the Rx filter 40 is connected while coupled to the first resonators FBAR14 though FBAR16 in parallel
As such, the first resonators FBAR11 through FBAR13 and the second resonators FBAR21 through FBAR22 are incorporated in series and in parallel so as to constitute a ladder type filter forming the Tx filter 50. Similarly, the first resonators FBAR14 through FBAR16 and the second resonators FBAR23 through FBAR26 are incorporated in series and in parallel so as to constitute a ladder type filter forming the Rx filter 40. Accordingly, the related art duplexer 10 functions as a band pass filter which filters signals in specific frequency bands. for the transmission and the reception.
Since the related art duplexer 10 functions to properly separate the signals transmitted and received via one antenna, it is necessary to avoid interference between the transmitted signal and the received signal in order to improve its performance. In further detail, since the frequency difference between the signals transmitted and received through the Tx filter and the Rx filter is very small, the duplexer is prone to, and sensitively responds to, interference. Therefore, it is required to avoid the interference between the transmitted signal and the received signal.
For doing so, an isolation part is required to prevent the interference by isolating the Tx filter 50 and the Rx filter 60. Thanks to the presence of the isolation part, the performance of the related art duplexer 10 can be enhanced because the interference and the noise insertion are avoided.
The isolation part is typically implemented as a phase shifter 70, which is interposed between the antenna port 20 and the Rx filter 60, using a capacitor and an inductor to make the frequency phase difference of the transmitted and received signals 90 degrees. Consequently, the isolation part can block the interference.
When the phase shifter 70 is not used and the Rx frequency band ranges 1.88˜1.95 GHz by way of example as shown in FIG. 12, impedance of the Rx filter 60 to the Tx filter 50 is close to zero. Accordingly, the Tx filter 50 and the Rx filter 60 are electrically connected and the Tx filter 50 is coupled and operated by the received signal.
By contrast, when the phase shifter 70 is interposed between the antenna port 20 and the Rx filter 60 and the Rx frequency band ranges 1.88˜1.95 GHz by way of example as shown in FIG. 13, impedance of the Rx filter 60 to the Tx filter 50 is close to infinity. Accordingly, the Tx filter 50 and the Rx filter 60 are in a line-open state and electrically isolated from each other, and the Tx filter 50 is prevented from being coupled and operated by the received signal.
As such, the Tx filter 50 and Rx filter 60 serve as the duplexer which shares a single antenna by filtering only Tx signal and Rx signal of the filtered frequency band by means of the phase shifter 70.
As discussed above, the isolation part of the related art duplexer 10 is constructed by incorporating the capacitors and the inductors between the antenna port 20 and the Rx filter 40. Thus, when the related art duplexer is implemented as a single chip set, there is a disadvantage in that a process margin is deteriorated because the number of processes increases for the implementation of capacitors and inductors on the substrate in addition to the fabrication of the FBARs. Moreover, another disadvantage is that an assembly cost of the duplexer rises because of the increased number of processes and the implementation cost of the circuit elements.